A grain flow measuring device of this type is known from German Patent Disclosure DE 37 31 080 C2. There, a long, narrow sensor plate is equipped with a piezoelectric oscillation sensor approximately in the center of its back, from which the connecting lines are routed to an amplifying circuit which is provided with a housing and is bolted to the sensor plate. From there, a connecting line is routed to an external electronic evaluation device in a multiple folded edge along the long side of the sensor plate. Along the folds on its long side, the sensor plate is bolted to retainer straps, which are fastened by means of rubber-metal connections at the outlet side of the sieve-like conveying surfaces of a rocker tray. In this way a leakage grain flow is detected by means of the sensor extending over the entire width of the rocker trays. Thereafter, the oscillation signals piezoelectrically detected at the impact of the grains are sequentially amplified, rectified, filtered, supplied to a threshold value comparator and then to a counter.
The elongated and undamped design of the sensor plate and the fastenings of the retainers and cables on it have the disadvantage that, depending on the place of impact of the grains and on the type of reflection on the edges and fastening points, very different signals occur. Not all of them get through the amplification and rectification and the threshold value discriminator and are therefore not fully detected, so that there is a relatively large uncertainty of measurement. Also, a considerable loss of grain is caused by the disposition of the sensor plate behind the end of the rocker. Furthermore, in its installed position behind the end of the rocker, the sensor plate is very much affected by subsequent processing of the straw, because some of the small pieces of chaff are hurled at great speed against the impact surface of the sensor, which generates faulty signals.
Furthermore, the disposition of a sensor plate with a piezoelectric pick-up on the output side of the end of a rocker is known from European Patent Disclosure EP 93 991 B1. In this case a grating is disposed extending past the rocker end, which is intended to keep the outflowing straw away from the impact surface. However, the grains falling on the sensor surface are lost.